Supercritical fluids are currently being investigated for semiconductor wafer cleaning. Supercritical fluids possess liquid-like solvating properties and gas-like diffusion and viscosity that enables rapid penetration into crevices and boundary layer films with removal of organic and inorganic contaminants contained therein. A key property of a supercritical fluid is that above a critical temperature, condensation cannot occur at any pressure. A phase diagram for CO.sub.2 is shown in FIG. 1. Above this critical temperature, the fluid will either be a gas (if the pressure is below the critical pressure) or a supercritical liquid (if the pressure is above the critical pressure). For CO.sub.2, the critical temperature is 31.degree. C. and the critical pressure is 73 bar (1050 psi).
One prior art method of supercritical fluid extraction involves cycling the pressure of the cleaning chamber between supercritical and subcritical values. CO.sub.2 pressurized at approximately 800 psi is supplied to the cleaning chamber. During cleaning, the CO.sub.2 fluid is pulsated by a hydraulic mechanism and the cleaning chamber is constructed such that pulsating the fluid changes the chamber height. The pressure of the supercritical fluid is varied according to the volume of the cleaning chamber. The pressure is cycled between 800 and 1200 psi at a frequency of 25 to 50 Hz. During the compression cycle, the cleaning fluid (in a supercritical state) penetrates crevices and molecular layers, as shown in FIG. 2a. During the expansion cycle, the cleaning fluid changes to a subcritical gas. Density decreases significantly. This can cause rapid mixing actions and outflow of fluid and contaminants from the wafer surfaces, as shown in FIG. 2b. Dislodged contaminants may then flushed out of the system when new cleaning fluid is added.
One problem with pressure cycling for supercritical wafer cleaning, however, is that the location of the phase transition between gas and supercritical liquid is arbitrary and the motion of particles (i.e., contaminants) is random. This can lead to inefficient and/or incomplete removal of contaminants. Accordingly, there is a need for better control of particle motions/removal and thus, better control of the location of the phase transition between gas and supercritical liquid.